Cylindrical batteries usually have a cylindrical metal battery case with a bottom for housing a power generating element. The opening of the battery case is sealed by a metal seal plate or seal assembly.
The seal assembly has a valve mechanism for securing battery safety. In the event that an abnormal condition occurs in the battery and the internal pressure of the battery case increases to a predetermined value or more, the valve mechanism is actuated, thereby causing the gas inside the battery case to be released. This prevents the battery case from becoming cracked or the like.
FIG. 6 illustrates an example of a conventional cylindrical battery. A battery 100 is a lithium ion secondary battery, and a battery case 102 contains an electrode assembly 110 which is obtained by spirally winding a positive electrode 104 and a negative electrode 106, with a separator 108 interposed therebetween. The opening of the battery case 102 is sealed by a seal assembly 112.
The seal assembly 112 has a bottom plate 115 at the lowest part and has an external terminal plate 116 at the highest part. Between them are an upper valve plate 118, a lower valve plate 120, an annular PTC thermistor plate 122, and an annular insulating member 124. The upper valve plate 118 and the lower valve plate 120 have annular fragile portions 118a and 120a, respectively, so that when the internal pressure of the battery case 102 increases abnormally, a valve hole is formed in the center. Also, the bottom plate 115 of the seal assembly 112 has a plurality of gas vent holes 115a near the edge thereof. The external terminal plate 116 also has a plurality of gas vent holes 116a. 
With this configuration, when the internal pressure of the battery case 102 increases abnormally, the fragile portions 118a and 120a of the upper valve plate 118 and the lower valve plate 120 rupture, thereby creating a valve hole through which the gas inside the battery case 102 is released to the outside.
However, with the recent increase in the functionality of electronic devices, the capacities of batteries are also becoming increasingly higher. As a result, when an abnormal condition occurs in lithium ion secondary batteries, the amount of increase in the internal pressure of the battery case is also becoming increasingly larger. To address this problem, various proposals have been made on techniques for ensuring battery safety.
For example, PTL 1 proposes setting the area of gas vent holes of a seal member to 0.15 to 1.2 cm2 per battery capacity (Ah), in order to prevent explosion or ignition due to a sharp increase in the internal pressure of a non-aqueous electrolyte battery.